An articulated truck carrying paint thinner, sulphuric acid and other dangerous chemicals, has rolled on State Highway 2, Mangatawhiri.
A police spokeswoman said the truck was blocking the northbound lane of the stretch of road, just east of Pokeno.
Emergency services were called to the scene at 8.39am.
Six fire crews were responding to the crash, including a hazardous materials crew from Otara.
A fire service spokesman said spilled chemicals were pooling on the road and flowing into drains.
The driver has escaped from the cab and is being assessed by ambulance staff. No other vehicles were involved.
The NZTA has asked motorists to follow directions of emergency services staff and to expect delays.